The continuing assault against the more than 200 al Qaeda fighters consists of helicopter gunships and fixed-wing aircraft, the sources said.

Pakistani President Pervez Musharraf said helicopter gunships and artillery were used Thursday to bombard the area in Waziristan, a remote region in northwestern Pakistan. He said the area surrounded has a perimeter of about 12 to 15 miles (20 to 25 kilometers).

Musharraf did not name the al Qaeda figure believed to be surrounded, referring to him as a "high-value target."

The well-trained and well-equipped al Qaeda fighters are trying to prevent al-Zawahiri's capture, an official in the intelligence services and another in the interior department said.

"The net is there," Musharraf told CNN. "We see very strong dug-in positions. The houses there are almost forts, they are mud forts. And all of these forts are occupied."

Escape 'very difficult'

Asked if it was al-Zawahiri, Suetan said with a smile, "Maybe, maybe."

"It's not clear yet, because there's lots of casualties. They are resisting, they are fighting, they are in large numbers," Suetan said.

He said the Pakistani forces were committed to "finish off the terrorists" and that within a day or two there might be "good news" to report. "You have to wait and see," he said.

Government sources said 18 al Qaeda operatives and some tribesmen were captured two days ago, and the information they provided suggested al-Zawahiri was in the area.

The government sources said the air assault would attempt to prevent al-Zawahiri from escaping.

The sources said they were concerned the fighters could try to use the cover of darkness to flee across the border. While Pakistani forces are trying to seal off the area, they cannot not do so completely, the sources said.

Pakistani information minister Sheikh Rashid Ahmed said if Zawahiri is in the region, it would be "very difficult for him to escape."

He said he expected the military operation to last another day or two.

Suspected 9/11 role

The 52-year-old al-Zawahiri, an Egyptian national, is considered to be Osama bin Laden's closest adviser and is viewed by many analysts as the operational brains behind the al Qaeda terror network.

U.S. officials believe bin Laden may be within miles of al-Zawahiri.

Al-Zawahiri is one of the most-wanted terrorists in the United States. U.S. officials believe if he is captured, they could likely work out a deal with Pakistan to have him turned over to U.S. custody.

He was indicted along with bin Laden for his alleged role as mastermind of the 1998 bombings of the U.S. embassies in Kenya and Tanzania that killed 224 people. (Full story)

U.S. government sources also believe he played a significant role in the September 11, 2001, attacks in Washington and New York.

National security adviser Condoleezza Rice said al-Zawahiri's capture would be a "major step forward," but she emphasized there is an entire network to break down, "not just one person."

U.S. officials said the United States was providing reconnaissance information to Pakistan, but they stressed it was a Pakistani operation.

The surveillance equipment included unmanned Predator aircraft armed with thermal cameras that can spot a person by their body heat.

White House communications director Dan Bartlett said he could not comment on whether there was U.S. involvement in the military operation.

Musharraf said it was the "fierce resistance" that troops encountered when they entered the area two days ago that led them to believe a top official with the terrorist network was being protected.

"They are not coming out in spite of the fact that we pounded them with artillery," he said.

Asked if he believed bin Laden or al-Zawahiri was hiding there, Musharraf demurred.

"I'm not going to say that because my previous experience is whenever I say it, then headlines come that he says, 'Al-Zawahiri is there,' or something like this," he said.

The "pitched battle" has already claimed the lives of some Pakistani troops, he said.

One U.S. official who is in touch with Pakistani officials about the situation told CNN that Pakistani forces were "fighting like hell."

The military asked locals to leave, flew helicopters overhead and pounded the area with artillery, he said.

'Finish the terrorists'

Word of the standoff came after Pakistan announced it had launched a fresh offensive against suspected militants near the Afghan border.

Hundreds of Pakistani troops backed by heavy artillery and helicopter gunships raided homes in the nation's tribal region of South Waziristan, two days after a fierce assault in the same area left dozens dead.

On Tuesday, at least 39 people were killed in a raid on suspected Taliban and al Qaeda militants in a fortress-like compound in Kaloosha, close to the border.

Fifteen soldiers died and Pakistani forces killed 24 suspects, most of them foreign fighters, military officials said. (Full story)

In retaliation, angry tribesmen torched more than a dozen military vehicles -- some loaded with ammunition -- on Tuesday and Wednesday.

Ahmed said Thursday that "for the first time in the history, Pakistani forces have entered there to finish the terrorists."

"We are committed against terrorism and we have to pay the price," he said.

Pakistan forces have launched a number of sweeps for "suspected foreign terrorists" along the border after Afghan and U.S. officials complained they were escaping to sanctuaries in Pakistan.

About 70,000 Pakistan troops are in the tribal regions, and the recent offensive coincides with a major U.S. military operation on the other side of the border in Afghanistan to capture terror suspects.

Though a spring offensive across southern and eastern Afghanistan, called Operation Mountain Storm, is yet to be officially launched, U.S. military operations there have been stepped up.